1. Field of the Invention
The present invention relates to an electrical connection structure between an electrical device such as a liquid crystal panel and an external derive circuit, and an electrically connecting method.
2. Related Background Art
For the electrical connection between an electrical device such as a liquid crystal panel having an electrical connection member and an external drive circuit, the method such as shown in FIG. 1 is well known. With this method, a film carrier tape has a copper foil pattern or film carrier electrodes 2 formed on a flexible insulating film 1 connected to a driver circuit (not shown). ITO electrodes 5 are formed on a cell glass 6 of a liquid crystal panel. An adhesive agent having metal particles or metal plated resin particles dispersed therein, forming an anisotropic conductive film 4, is placed between the film carrier electrodes 2 and ITO electrodes, and both the electrodes are thermally compressed by using a thermocompresssion tool 7 for the thermocompression bonding, thereby achieving both mechanical and electrical connections.
Namely, the film carrier electrodes 2 from the external drive circuit and the device electrodes 5 are aligned with each other. The anisotropic conductive film 4 is placed at the necessary positions between both the electrodes. The thermocompression tool 7 or the like heated to a desired temperature is used to thermally compress and bond for achieving both the mechanical and electrical connections.
With the above-described technique, however, the positions of the film carrier electrodes 2 on the flexible film 1 are shifted from those of the panel electrodes 5 because of thermal expansion of the film 1 during the thermocompression bonding, as shown in FIG. 2.
Specifically, a flexible film is generally made of organic material so that it expands when heated. The thermal expansion of a flexible film is generally greater than that of metal. Therefore, the same pitches of the panel electrodes and flexible film electrodes before heating become different because of the thermal expansion of the flexible film during heating.
This phenomenon poses no problem if the pitches are large and the density of electrodes is small (i.e., if wide electrodes are formed at sufficiently large intervals) and if the length of the electrical connection area is short (if the length of the electrically connected electrodes in the longitudinal direction of the film is short).
However, for recent liquid panels, photoelectric conversion element arrays (sensor arrays), recording element arrays such as ink jet recording element arrays and thermal recording element arrays, it has been desired that the electrode area should be as small as possible, and/or that the number of electrodes should be increased as great as possible. In this context, the pitches of electrodes become small and dense, and in some cases the width of the electrical connection area becomes wider, i.e., the number of electrodes increases.
The position or pitch shift of electrodes is therefore likely to occur because of thermal expansion of a flexible film as discussed above.
This pitch shift results in not only defective connection but also in lower mechanical connection strength or damages of the device itself or peripheral circuits caused by overlooking such defective connections.
In the example shown in FIG. 2, the center line of the device electrode connection area is aligned with that of the flexible film.
As shown in FIG. 3, some electrical connections use an overhang structure in which a copper foil 2 is exposed by removing the film carrier flexible film at the electrode connection area in order to eliminate the influence of thermal expansion of the film 1, with an anisotropic conductive film 4 being used for the mechanical and electrical connections. With this overhang structure, in order to improve the connection reliability, particularly electrical connection reliability, it is necessary to ensure, e.g., about twenty particles per one connected electrode area. It is therefore necessary for the connected electrode area to have a length of 3 mm or longer. In this case, the length a of the exposed electrode becomes 4 mm or longer with a clearance for allowing a thermocompression tool 7 to fit in the exposed electrode area. Even the overhang structure shown in FIG. 3 has a problem to be solved in order to reduce the length of the exposed electrode area.
A film carrier having outer leads finer than a 100 .mu.m pitch requires necessarily a pattern of inner wirings of about 80 .mu.m or less. In this case, it is desirable to use a thin copper foil of about 18 .mu.m thickness for example in order to obtain a good etching property of copper foil. As shown in FIG. 4, such a thin copper foil exposed electrode has an insufficient strength relative to a force applied perpendicular thereto. In some cases, there occurs the problem that the exposed electrode 3 is bent by cured resin flown from the anisotropic conductive film. Namely, when the thermocompression tool 7 is used, the anisotropic conductive film is pressed, and resin before curing moves to apply a force to the exposed electrode 3 and bend it.
Such bending leads to incorrect electrical connection, as in the case of the pitch shift. If the device is used and such incorrect electrical connection is overlooked, electrical shortage may occur, resulting in possible damage of the device itself and peripheral circuits.
Similar to FIG. 2, in the example shown in FIG. 3, the center line of the device electrode connection area is aligned with that of the flexible insulating film.